In recent years, memories of the next generation, such as a ferroelectric random access memory (FeRAM), a magnetic random access memory (MRAM) and a resistance random access memory (ReRAM), have been proposed. In particular, a ReRAM, which includes a plurality of memory cell arrays stacked on a substrate in a three-dimensional structure, has been proposed (see, e.g. US2008/0258129). According to this ReRAM, the chip area can greatly be reduced.
In the above-described ReRAM, a transition metal oxide is used, in some cases, as the material of memory cells. In order to set the transition metal oxide in a resistance-variable state, it is necessary to perform a forming operation which is an initializing operation. However, if the forming operation is performed only once, the resistances of the memory cells in the memory cell arrays are not made uniform in the same state. To address this problem, for example, a method has been thought of performing the forming operation more than once, thereby gradually setting the resistances of memory cells of the memory cell arrays in a low resistance state and a high resistance state. However, the voltage for the forming is high, and if the forming is performed many times, the memory cell array may possibly be destroyed. In addition, even if the forming is performed, it is possible that the memory cells are not set in the state (proper resistance state) in which the memory cells can surely perform operations. Thus, in the conventional forming method, it is difficult to form a ReRAM using a transition metal oxide, which can perform a highly reliable memory operation with high quality.